This is a competingcontinuation application that proposes to study the activation receptors on natural killer (NK) cells whichare important constituentsof the early host innate immune response to tumors and infections. Inthe current funding period, the applicant's laboratory discovered a new high affinity ligand (MULT1) for the NKG2D activation/co-stimulatory receptor that otherwise recognizes ligands whose expression is up-regulated by stress-induced transcription. This "induced-self recognition appears to be a key component of NKG2D function. Transcripts for MULT1, however, are constitutively expressed, suggesting that MULT1 expressionmay be regulated in another manner. Moreover, the applicant's laboratory also identified new ligands (Clr) for the Nkrpl family of NK cell receptors. Whereas most ligands for NK cell receptors are related to major histocompatibility complex (MHC) class I molecules, these ligands are lectin-like,suggesting a novel MHC-independent mechanism for controlling NK cell activity. However, the ligand is unknownfor the most prominent member of the Nkrpl family, NK1.1 (Nkrplc), an activation receptor that is the most specific serologic marker on CD3-neg NK cells in C57BL/6 mice. Finally, the genes for the Nkrpl receptorsand Clr ligands are co-localized in the genome in an area with suppression of recombination. Related genetic strategiesto keep paired receptor-ligand genes together have been described in plants, termed the self-incompatibility locus, implying the evolutionary importance of keeping the receptor and ligand genes together. Thus, the specific aims of this proposal are to study: 1) MULT1 and the induced-self model of NKG2D function. Experiments will seek to determine the mechanism underlying potential regulatedexpression of MULT1. 2) Non-MHC specific NK cell receptors. Experiments will further define the specificity and function of the Nkrpl family. 3) Genetic protection of Nkrpl and Clr genes. Experiments will explore the evolutionary conservation of receptor-ligand interactions. Thus, these studies will provide fundamental new insight into the function of NK cells and their specificities.